Computed Tomography (CT) scanners are widely used in human and veterinary medicine, small animal scanning, industrial applications and homeland security. These scanners produce images of a subject by reconstruction of X ray attenuation data acquired over multiple view angles. Typically, a radiation source is rotated about the scanned subject and the X ray beam attenuated by the subject is measured by a detector array disposed opposite the source. Cross sectional or 3D images of the scanned subject are reconstructed from the attenuation data by algorithms known in the art as filtered back-projection or by other reconstruction methods.
Image reconstruction methods known in the art require attenuation data over at least a minimal angular range of the source relative to the subject. For example, filtered back-projection requires data over a continuous angular range of at least 180°+fan angle, where the fan angle is the beam angle covering the desired scan field of view.
Early generation CT scanners had a one dimensional detector array and were capable of scanning one axial slice of the subject at a time. More recent CT scanners have a two dimensional detector array comprising multiple rows of detector elements. These scanners, usually referred to as multislice or multidetector CT scanners, are capable of scanning multiple substantially parallel slices of the subject simultaneously. Further, CT scanners with a large number of detector rows are typically referred to as cone beam scanners. Cone beam scanners image a whole volume at a time.
Some CT scanners use a “step and shoot” protocol. In this protocol the gantry rotates about a stationary subject to generate a single or multiple axial images of the scanned subject, the subject is translated relative to the gantry, the gantry rotates again to generate images of an adjacent region, etc. Other CT scanners use a helical or spiral mode wherein the subject is being translated relative to the gantry while the gantry rotates and attenuation data is acquired.
Since CT scanning takes time, scanning subjects which move during the scan, may result in image blur and artifacts due to the subject's motion. Motion effects can be reduced by making the gantry rotation and data acquisition faster. Electron beam CT using electromagnetic steering of the X-ray source position rather than mechanical rotation, achieve even faster scan time.
Of particular interest are subjects which have periodic motion such as the human heart. The heart not only returns to approximately the same position every cycle, it also has particular phases in the cycle (e.g. late diastolic phase) in which the motion is minimal. Several solutions or combinations thereof are known in the art for CT imaging of the heart with motion freeze:                a. Spiral scan with retrospective gating—the source rotates about the subject at high rotation speed while the subject is translated axially at a relatively low pitch. X ray attenuation data and ECG data are acquired over multiple heart beats. The data is sorted after the scan and only data from ECG phases of minimal motion are used for reconstruction.        b. Spiral scan with prospective gating—same as above except the subject is translated axially at a very high pitch so that the entire heart is covered within a fraction of a heart beat. The one scanner available commercially that is using this mode (Siemens Definition Plus) is using dual sources and detectors to achieve the required coverage in a short time.        c. Axial scan with prospective gating—covering the required angular range in a single rotation and a single X ray shot gated by the ECG. The source rotation must be fast enough in order to be able to cover the entire angular range for image reconstruction within a fraction of a heart cycle. Data from multiple shots over consecutive heart beats may be added for improved statistics (e.g. in electron beam CT). Several shots over consecutive heart beats with patient translation between the shots may be applied to increase axial coverage (“step and shoot” protocol).        d. Multiple source rotations, each lasting multiple heart beats and acquiring non-continuous angular sectors of data during the desired heart phase, such that the multiple rotations provide together full angular coverage. ECG gating may be applied retrospectively or prospectively. The timing and speed of the rotations may be optimized according to heart rate.        
Methods a through c above require a fast rotation high power CT scanner, which is expensive and costly to operate. Some of these methods apply more radiation dose than desired. Method d can be applied on a lower cost slower rotation scanner but the acquisition tends to take a long time. Considering for example a Siemens Artis C-arm operated in ECG gated “DynaCT” mode. The arm makes four 220° rotations of 5 sec each to acquire a total of 220° gated data in the desired heart phase. Considering about 2 sec for switching directions between rotations (the C-arm rotates back and forth), the procedure may take about 26 sec. In addition to heart motion, cardiac imaging is also sensitive to breathing motion, so cardiac imaging is preferably done during a breath stop. Many patients cannot hold their breath for 26 sec so the procedure is not applicable to them. In addition, the protocol is sensitive to irregularities in heart rate (arrhythmia).
Therefore, there is a need for a better apparatus and method for scanning the human heart or other periodically moving subjects.